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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

SILVER ALLOYS

279

greatest accuracy.1 These are wrapped in small pieces of white paper or
thin lead foil, the weight of which is allowed for in calculating the amount
of lead to be used, and placed on a tray consisting of a sheet of copper pro-
vided with a handle and stamped into cavities to take the test pieces. Be-
side each button is placed the necessary quantity of lead.

The cupels are placed in the muffle and close to them the thermo-electric
couple,2 the temperature being then raised to bright redness, that is, to
about 950. When these have assumed the temperature of the muffle
(indicated by the absence of a dark zone between the bottom of the cupel
and the base of the muffle), the pieces of lead are introduced into the cupels
by means of suitable tongs. The lead at first melts and becomes covered
with a layer of oxide and after some time uncovers, that is, assumes a shiny
appearance.

When the lead is uncovered, the test pieces are placed in the cupels with
great care to avoid loss by projection, the door of the muffle being left open
a little to permit of observation and to give access to the air. The test
pieces melt and a shining appearance is resumed. Over the surface of the
fused metal, which is at first only slightly convex, luminous points are seen
to run and become absorbed by the cupel. As the cupellation proceeds the
convexity increases and the drops of fused litharge, of oily appearance,
become larger and circulate more rapidly. At this point the temperature
should be raised a little by closing the door of the muffle and increasing the
draught of the furnace, in order to oxidise the last particles of lead and
keep the button of silver fused. As the last portions of lead " pass " from
the silver, the molten metal, which is in a state of considerable agitation,
exhibits a kind of iridescence, this soon disappearing", the button then
appears opaque and still, but suddenly flashes out brightly. This indicates
the end of the operation.

The cupels are then gradually brought near to the door of the muffle
so that the buttons of silver may cool slowly and rapid release of the occluded
oxygen (fused silver absorbs up to 22 volumes of oxygen) not give rise to
projection (spitting or vegetating] of the metal. After a few minutes the
cupels are withdrawn from the muffle and the metallic buttons detached,
hammered slightly on both sides, held in tongs and freed with a scratch-
brush from the adherent cupel dust and weighed.

The total weight of the two test pieces in milligrams, if these were each
of 0-5 gram, or this weight divided by two, if the samples were i gram each,
gives the fineness of the alloy.

If the cupellation is successful, the silver buttons obtained from the

1 "With samples of silver and gold, to obtain the highest accuracy and to be able
to weigh directly, very sensitive balances are employed with an exactly equally divided
long beam and with very small movable dish-shaped pans, on which the test pieces
are placed directly. The maximum load of such a balance is 2-3 grams.

z If no thermo-electric couple and corresponding voltmeter-pyrometer are available,
the temperature of the furnace during cupellation may be regulated by observation
of the way in which the fumes of litharge are evolved. When the temperature is
suitable, the fume rising from the tests should reach only half-way up the muffle and
small, lamellar crystals of litharge should be seen on the edge of the cupel. If the
temperature is too low, the fume licks round the edges of the cupel, whilst, if too high,
it rises rapidly towards the crown of the muffle.l and ether, dried at